How can I classify the features in a grayscale image into different classes?

I have a grayscale image of a village. I need to divide the image into three classes- constructed areas, open land and roads. I have tried using K-means clustering, Fuzzy c means clustering and watershed transform, but did not get satisfactory results. This is my first project with Matlab and I think i might be unaware of some features that might be helpful. Can anyone suggest any function or code to do this.

You can find the image here: http://imgur.com/6dYqj7v